For the coordinate measurement on workpieces, coordinate measuring apparatus having measuring or switching probe heads are, as a rule, utilized. Such apparatus directly measure the workpiece surface by touching mechanical contact with the probe element attached to the deflectable part of the probe head. The probe element is, in most cases, a ball.
In addition, coordinate measuring apparatus are known which carry a so-called optical probe head which "touches" the object to be measured without contact. As optical probe heads, in addition to probes operating with light points (such as triangulation probes), mostly so-called video probe heads are utilized, that is, video cameras which detect a greater portion of the workpiece surface. The coordinates of geometric elements of the workpiece of interest are determined then, for example, via a video processing software.
In recent times, combined optically and mechanically contacting coordinate measuring apparatus have become known. One such apparatus is described, for example, in U.S. Pat. No. 4,908,951. The known apparatus has two spindles of which one spindle carries the mechanical probe head while the other spindle holds a video camera having a beam path into which a laser probe is simultaneously reflected for making measurements in the z-coordinate, that is, along the optical axis of the video camera.
Such combined optically and mechanically contacting coordinate measuring apparatus permit not only the measurement of different workpieces such as printed circuit boards or motor housings with the measuring methods best adapted thereto (optically without contact or mechanically contacting). It is also possible to measure locations on the same workpiece first with the video camera and then with the mechanical probe since the mutual reference of the two probe systems can be established via a calibration procedure.
The actual measuring procedure then takes place as described below.
(a) Manually controlled operation: Here, the location on the workpiece to be contacted with the mechanical probe head is approached under direct visual control. After contacting the workpiece with the probe element, which is mostly indicated acoustically, the computer assumes the coordinates of the contact point and the operator drives with the probe element to the next measurement location on the surface of the workpiece. While measuring with the video camera, the video camera is moved over the corresponding location on the workpiece and focusing of the image of the workpiece surface is monitored with the aid of the monitor and thereafter, the transfer of data is triggered. PA1 (b) CNC-operation: Here, the positions to be approached by the mechanical probe head or by the video camera are either in part remotely programmed or, in the context of a so-called learning program, the movement sequence is carried out on a sample piece including driving to the contact locations for the special measuring program, then stored and then repeated as often as desired in the series measurement operation on a series of like parts to be measured.
The manually controlled contacting presents problems especially for complicated workpieces having poorly accessible inner surfaces because it is difficult to maintain visual contact with the probe element. If this visual contact is lost, then the danger is present that the probe head or probe pin is damaged by a collision with the workpiece or the incorrect location on the workpiece is contacted. This danger is present also during the learn programming of the CNC-operation since the probe element is there likewise manually-controlled driven.
From East German Patent 12,609, it is known to measure bores with the aid of an illuminating measuring bead under visual control. The measuring bead is brought mechanically into contact with the bore wall. The method described in this East German patent is, however, not applicable to a coordinate measuring apparatus and the measuring bead is not a mechanical probe such that this probe can independently register the coordinates of the contact point. Instead, the measuring bead simply serves as a measuring mark with the aid of which the bore diameter is determined visually with an ocular plate in a measuring microscope. The method can furthermore only be used for bores which are accessible from both ends.